1. Field of the Invention
The present disclosure relates in general to a programming method. In particular, the present disclosure relates to a programming method for electrical fuse cells and a circuit using the method.
2. Description of the Related Art
Fuse links are frequently used in conjunction with redundant memory cells of memory ICs or embedded memory segments in application specific integrated circuits (ASICs) or system level integrated circuits (SLICs). If a memory cell is defective, a redundant memory cell is substituted and the defective memory cell is disconnected. It is not unusual for IC memory chips or memory segments of ASICs or SLICs to be formed by hundreds of thousands, or millions, of such memory cells. With such a large number of memory cells, there is a significant risk that at least some of those memory cells will be defective when fabricated. Using programmable fuse link structures in memory segments of ASICs or SLICs, as well as in other parts of the IC, increases economic efficiency in IC fabrication by substantially raising the yield of functional circuits produced from the fabrication process. In addition, fuse links are also applied to chip ID setting and resistor trimming in chip design implementation.
The typical method of blowing, severing or opening a conventional fuse link involves focusing a laser beam on a fuse structure formed in the IC. The energy density and pulse duration of the laser beam deliver sufficient energy to the fuse link to vaporize or melt the fuse material, thereby severing or opening the pre-existing electrical path through the fuse link. Once the electrical path has been opened, current can no longer flow through it. The circuitry of the IC has been designed to respond to this open circuit by disconnecting defective circuit elements and inserting correctly functioning circuit elements.
U.S. Pat. No. 6,566,730 to Giust et al. discloses laser-breakable fuse links with alignment and break point promotion structures.
However, the large energy of the laser beam presents the possibility of thermal damage to adjoining circuitry of the IC. In addition, the ability to precisely position the laser beam is important, because a directly positioned laser beam will be more effective in melting the fuse link. Another consideration is to provide the smallest possible spot or diameter of a laser beam. Thus, the density of structures on the semiconductor device may be limited. To provide laser beams with precise location and appropriate diameter, the cost of the laser beam generator is uneconomical.
Another type of fuse link is an electrical fuse. U.S. Pat. No. 6,661,330 to Young et al. discloses electrical fuses for semiconductor integrated circuits. The electrical fuse has recently been emphasized as a means of replacing metal fuses to reduce testing and programming costs.
FIGS. 1A and 1B illustrate a top plane view and a cross-section, respectively, of a portion 10 of an integrated circuit (not shown) comprising a conventional fuse 15 prior to programming. FIG. 1A illustrates the fuse 15 formed over an insulation layer 12, wherein the fuse comprises two contacts 13A and 13B in electrical contact with an electrically conducting silicide layer 14. As illustrated in cross-section in FIG. 1B, the silicide layer 14 is disposed over a polysilicon layer 16, wherein the silicide layer 14 and the polysilicon layer 16 are generally arranged in a stack residing over the insulation layer 12. Typically, the insulation layer 12 is an oxide layer deposited or grown on a semiconductor substrate 18, such as monocrystalline silicon. Furthermore, the fuse 15 is generally covered with an insulative passivation layer 17 to electrically isolate the fuse from other devices (not shown).
During programming and operation, electrical current through the fuse 15 generally proceeds from one contact 13A, through the silicide layer 14, to the other contact 13B. If the current is increased to a level that exceeds a predetermined threshold current of the fuse 15, the silicide layer 14 will change its state, for example, by melting, thereby altering a resistance of the structure. Note that depending on the sensitivity of the sensing circuitry (e.g., a sense amp), a fuse may be considered “blown” if a change in resistance is only modest. Therefore the term “blowing” a fuse may be considered to broadly cover a modest alteration of the resistance or alternatively, may comprise a complete open circuit. FIG. 1C illustrates a cross section of the fuse 15 shown in FIG. 1B after the fuse has been programmed (e.g., a “blown” fuse), wherein the programming current has effectively melted or otherwise altered a state of the silicide layer 14 in a region, thereby forming a discontinuity 11 in the silicide layer, wherein agglomerations 19 of silicide are formed on either side of the discontinuity.
The programming of conventional electrical fuse requires I/O driver device with operating voltage in the range of 2.5V˜3.3V, unlike the operating voltage of the core device. Thus, a level shifter is generally required to bridge the I/O drive device from the core device. In addition, to sustain the large programming current, the transistor of the conventional I/O drive device comprises a gate oxide thicker than that of the core device, thereby generally requiring additional process steps and mask layers for I/O driver device.